1. Field of the Invention
The present invention relates to wireless communications, and, in particular, to receivers for multiple-input, multiple-output (MIMO) systems, such as MIMO orthogonal frequency division multiplexing (OFDM) wireless local area network (WLAN) systems.
2. Description of the Related Art
In a conventional M×M MIMO-OFDM WLAN system, a transmitter with M transmit antennas transmits M OFDM signals (a type of multi-carrier signal) to a receiver having M receive paths that are used to generate M recovered versions of the transmitted OFDM signals.
FIG. 1 shows a block diagram of the front end of a prior-art receiver 100 for use in a conventional 2×2 (i.e., M=2) MIMO-OFDM WLAN system. As shown in FIG. 1, 2×2 receiver 100 has two receive paths 102 and a processor 104. Each receive path 102 has a receive antenna 106, an RF/analog block 108, an analog-to-digital converter (ADC) 110, a cyclic-prefix (CP) removal block 112, and a fast Fourier transform (FFT) block 114.
Within each receive path 102, receive antenna 106 receives mixed versions of the M OFDM signals transmitted by a two-antenna MIMO-OFDM WLAN transmitter. In RF (radio frequency) implementations, RF/analog block 108 downconverts the analog RF OFDM signals from receive antenna 106 to an intermediate frequency (IF) or baseband, and ADC 110 digitizes the resulting downconverted analog OFDM signals to form a digital stream consisting of OFDM symbols separated by cyclic prefixes. CP removal block 112 removes the cyclic prefixes from between the OFDM symbols in the digital stream. FFT block 114 converts frames (e.g., 20 msec) of digital OFDM symbols in the time domain into frequency-domain coefficients for different OFDM subcarrier signals.
Processor 104 has a matrix 116 for each subcarrier in the multi-carrier OFDM signals. Each matrix 116 receives, from the different FFT blocks 114, all of the coefficients for one of the OFDM subcarriers. Note that the data from each FFT block 114 typically contains information for OFDM signals transmitted from all of the transmit antennas. Each matrix 116 is adapted to process the set of corresponding subcarrier coefficients to recover separate and equalized subcarrier coefficient streams 118, each different recovered subcarrier coefficient stream 118 corresponding to the OFDM signal transmitted from a different transmit antenna. Although not shown in FIG. 1, the recovered OFDM subcarrier coefficient streams 118 can then be processed using conventional OFDM decoding techniques, e.g., to detect the data encoded in the received OFDM signals.
One problem with MIMO-OFDM WLAN systems relates to co-channel interference (CCI), where a receiver simultaneously receives signals from spatial locations other than those of the transmitters of interest. CCI can occur when one or more additional transmitters operate at the same channels (e.g., using the same OFDM subcarriers) as the transmitters of interest, such that signals from the additional transmitter(s) interfere with the recovery of the desired signals from the transmitters of interest. Another scenario of CCI occurrence may be due to poor channel selectivity filtering of the receiver, where adjacent channel signals fold into the channel of the interest.